This invention relates generally to cable converters and, specifically, to cable converters equipped to process a television signal transmission having a stereo audio signal.
Recently, "stereo television", that is a television signal with two separate audio channels, has been made available. The additional audio information is included on a separate subcarrier of the 4.5 MHz aural intercarrier in the conventional NTSC television signal. Properly equipped television receivers are capable of decoding the additional information on the aural intercarrier and, by further processing, for producing separate audio information channels.
A large number of television receivers are employed in cable-connected systems where television signals are received via a coaxial cable from a so-called "head-end", as distinct from being received as over-the-air transmissions. The cable systems include cable converters for receiving television programming carried on a plurality of channels, selecting a desired one of the channels, demodulating the television program signal thereon and remodulating it onto a fixed carrier of VHF channel frequency, normally corresponding to VHF channel 3 or VHF channel 4 (channel 3/4). The converter generally includes apparatus and appropriate circuitry for permitting the subscriber to remotely control the channel selection mechanism in the converter and to exert control over the volume level of the audio information in the received television signal. The converter is also controllable from the cable head-end by means of appropriate built-in signalling circuitry. Thus any converter may be selectively enabled to receive all channels or to not receive one or more so-called "premium" channels, depending upon the level of service the subscriber has requested and paid for. The converter can thus be "authorized" for regular or premium channels by means of an authorization signal transmitted from the head-end to that individual converter. Thus, the cable operator can offer his subscribers different levels of programming, with correspondingly different fee structures.
The advent of stereo TV, that is television with dual audio channel capability usually in the form of a stereo audio signal, proved troublesome for cable operators since subscribers may have "stereo television" receivers or want to take advantage of the dual channel audio capability of such television programs. Present design baseband converter/decoders do not include audio demodulators and RF remodulators of sufficient quality to provide proper stereo audio TV signals to subscriber home equipment.
Stereo audio adapter units are commercially available for accepting the 4.5 MHz detected aural intercarrier and its stereo signal, processing it and providing separate left and right channel audio outputs for driving separate speakers. One such adapter, marketed by Zenith Electronics Corporation under the model designation CV 524, includes a DIN type multi-pin jack for receiving, by means of a mating plug, appropriate signals for developing the stereo audio information from normal, that is, monaural (mono) television receivers that have been adapted to provide the appropriate signals. Appropriate kits are also available for adapting a mono television receiver for use with such a stereo audio adapter.
In copending application Ser. No. 649,396, filed Sept. 11, 1984, now U.S. Pat. No. 4,630,113, issued Dec. 16, 1986, in the name of Michael E. Long and assigned to Zenith Electronics Corporation, a very simple bypass circuit is disclosed for enabling a cable converter to "process" television signals having a stereo audio signal. In that application, a switch is provided on the converter for selecting or enabling either a "standard" audio path or a "bypass" audio path. With the bypass audio path enabled, the 4.5 MHz aural intercarrier is shunted around the converter decoding circuitry and made directly available at an output. The cable converter normally provides the user with remotely controllable audio as well as an audio mute function. A mute control, which also serves as an authorization control for the cable head-end, is retained when operating in the stereo mode by disabling the bypass path in response to a mute remote control signal from a user or a deauthorization signal from the cable head-end. The user does lose the ability to control the level of the stereo audio signal in a continuous manner, although he retains a mute control and the head-end retains an authorization control of the converter. That simple circuit may suffice for many installations.
A straight-forward solution to enable full utilization of the cable converter capabilities is to provide a plurality of terminals on the converter for interconnecting with corresponding terminals on the stereo adapter unit. Such a solution, in the situation mentioned for the CV 524 adapter unit, would necessitate installing a multiple contact DIN jack on the converter and would prove quite expensive. Furthermore, subscribers who possess stereo television receivers would, in all likelihood, want to utilize the full capabilities of those television receivers and not be forced to purchase and use a separate stereo adapter unit. Thus, there is a need in the art for a cable converter that is capable of providing in a simple, cost-effective way, the stereo audio signal information in an appropriate television transmission.